The terror bird Andalgalornis brings its powerful beak down in a
hatchet-like jab to attack its prey, a peculiar cat-sized
herbivorous mammal called Hemihegetotherium.
Illustration by Marcos Cenizo, courtesy of the Museo de La
Plata.

Fossil skull of the terror bird Andalgalornis,
compared with the skull of a modern-day golden eagle and a
human skull for scale. Courtesy of Ohio University.

Skull of the terror bird Andalgalornis,
based on a volume-rendering of CT-scan data. Gray represents
the fossil, whereas lavender represents persistent rock and
plaster used to join broken pieces. Vertical lines show from
where in the beak the CT-scan cross sections below were
taken. Note the narrow and hollow beak which was very strong
in the vertical dimension but dangerously weak from side to
side. Stars show key areas of the skull that have evolved to
be more rigid, turning the mobile joints of other birds into
strong beams in the terror bird.Courtesy of WitmerLab at
Ohio University.

Biomechanical analysis of the skull of the
terror bird
Andalgalornis, based on the
engineering method of finite element modeling. Each image
represents a different simulation of predatory feeding
behavior, lower (safer) stress are shown in bluer colors
whereas higher (more dangerous) stresses are shown with
colors toward the red end of the spectrum. Left: The skull
was well-adapted (as shown by a largely blue skull) to hold
onto prey with the bill tip and tear off chunks of flesh by
pulling straight back with the neck. Middle: A killing bite
driving the beak straight down into the prey like a hatchet
is more stressful for the skull, but still is within safe
limits. Right: Shaking the beak from side to side, as when
thrashing small prey or when grappling with large prey, is a
very unlikely behavior because the stresses are so high that
fracture of the skull would have been likely. Courtesy of
the Museo de La Plata and the University of New South Wales.

Skull of the terror bird Andalgalornis,
based on a volume-rendering of CT-scan data. Gray represents
the fossil, whereas lavender represents persistent rock and
plaster used to join broken piecesCourtesy of WitmerLab at
Ohio University.

Fossil skull of the terror bird Andalgalornis. Courtesy of WitmerLab at Ohio
University.

Fossil skull of the terror bird Andalgalornis,
compared with the skull of a modern-day golden eagle for
scale. Courtesy of WitmerLab at Ohio University.

Article lead author Federico Degrange with
the terror bird Andalgalornis at the Field Museum of Natural History
in Chicago. Courtesy of the Museo de La Plata.

Ancient “terror bird”
used powerful beak to jab like an agile boxerInternational team uses computer modeling
to reconstruct kills of prehistoric predator

ATHENS, Ohio (Aug. 18, 2010)—The ancient “terror bird”
Andalgalornis couldn’t fly, but it used its
unusually large, rigid skull—coupled with a hawk-like
hooked beak—for a fighting strategy reminiscent of boxer
Muhammad Ali. The agile creature repeatedly attacked and
retreated, landing well-targeted, hatchet-like jabs to
take down its prey, according to a new study published
this week in the online, open-access journal PLoS ONE
by an international team of scientists.

The study is the first detailed look at the predatory
style of a member of an extinct group of large,
flightless birds known scientifically as phorusrhacids but popularly labeled “terror birds”
because of their fearsome skull and often imposing size.
Terror birds evolved about 60 million years ago in
isolation in South America, an island continent until
the last few million years, radiating into about 18
known species ranging in size up to the 7-foot-tall (2.1
meters) Kelenken.

Because terror birds have no close analogs among
modern-day birds, their life habits have been shrouded
in mystery. Now, a multinational team of scientists has
performed the most sophisticated study to date of the
form, function and predatory behavior of a terror bird,
using CT scanning and advanced engineering methods.

“No one has ever attempted such a comprehensive
biomechanical analysis of a terror bird,” said study
lead author Federico Degrange of the Museo de La Plata/CONICET
in Argentina, who is conducting his doctoral research on
the evolution of terror birds. “We need to figure out
the ecological role that these amazing birds played if
we really want to understand how the unusual ecosystems
of South America evolved over the past 60 million
years.”

The terror bird under study is called
Andalgalornis
and lived in northwestern Argentina about six million
years ago. It was a mid-sized terror bird, standing
about 4.5 feet tall (1.4 meters) and weighing in at a
fleet-footed 90 pounds (40 kg). Like all terror birds,
its skull was relatively enormous (14.5 inches or 37
centimeters) with a deep narrow bill armed with a
powerful, hawk-like hook.

Article co-author Lawrence Witmer of the Ohio University
College of Osteopathic Medicine ran a complete skull of
Andalgalornis through a CT scanner, giving the
team a glimpse into the inner architecture of the skull.
The scans revealed to Witmer, Degrange and article
co-author Claudia Tambussi, also from the Museo de La
Plata/CONICET and Degrange’s PhD advisor, that Andalgalornis was unlike other birds because it had
evolved a highly rigid skull.

“Birds generally have skulls with lots of mobility
between the bones, which allows them to have light but
strong skulls. But we found that Andalgalornis
had turned these mobile joints into rigid beams. This
guy had a strong skull, particularly in the fore-aft
direction, despite having a curiously hollow beak,” said
Witmer, Chang Ying-Chien Professor of Paleontology and a
professor of anatomy.

The evolution of this large and rigid bony weapon was
presumably linked to the loss of flight in terror birds,
as well as to their sometimes gigantic sizes.

From the CT scans, Stephen Wroe, director of the
Computational Biomechanics Research Group at the
University of New South Wales, Australia, assembled
sophisticated 3D engineering models of the terror bird
and two living species for comparison (an eagle, as well
as the terror bird’s closest living relative, the
seriema). Using computers and software supplied by Wroe,
Degrange and Karen Moreno of the Université Paul
Sabatier in Toulouse, France, applied an approach known
as Finite Element Analysis to these models to simulate
and compare the biomechanics of biting straight down (as
in a killing bite), pulling back with its neck (as in
dismembering prey) and shaking the skull from side to
side (as in thrashing smaller animals or when dealing
with larger struggling prey). Color images created by
the program show cool-blue areas where stresses are low
and white-hot areas where stresses get dangerously high.

The engineering simulations supported the CT-based
anatomical results. “Relative to the other birds
considered in the study, the terror bird was
well-adapted to drive the beak in and pull back with
that wickedly recurved tip of the beak,” remarked Wroe,
“but when shaking its head from side to side, its skull
lights up like a Christmas tree. It really does not
handle that kind of stress well at all.”

A
key part of the engineering analysis was determining how
hard of a bite Andalgalornis
could deliver.
To examine bite force in birds in general, Degrange and
Tambussi worked with zookeepers
at the
La Plata Zoo
to get a seriema and an eagle to chomp down on their
bite meter.

“Combining
all this information, we discovered that the
bite force of Andalgalorniswas a little lower than we expected and weaker than the
bite of many carnivorous mammals of about the same size.
Andalgalornis may have compensated for this
weaker bite by using its powerful neck muscles to drive
its strong skull into prey like an axe,” Degrange said.

Taken together, the team’s results give new insight into
the lifestyle of a unique avian predator. This terror
bird was no slugger and couldn’t wade into the fray like
a feathered Joe Frazier. Its skull, though strong
vertically, was too weak from side to side, and the
hollow beak was in danger of catastrophic fracture if Andalgalornis grappled too vigorously with large
struggling prey.

Instead, the study shows that the terror bird was
required to engage in an elegant style more like
Muhammad Ali, using a repeated attack-and-retreat
strategy, using well-targeted, hatchet-like jabs. Once
killed, the prey would have been ripped into bite-sized
morsels by the powerful neck pulling the head straight
back or, if possible, swallowed whole.

Feeding on a diversity of strange, now-extinct mammals
and competing with the likes of saber-tooth marsupials,
terror birds became top predators in their environment.
At least one gigantic terror bird, Titanis,
eventually invaded North America about two to three
million years ago, but the animals disappeared from
Earth shortly after.

The research was funded by grants to Witmer from the
U.S. National Science Foundation; to Wroe from the
Australian Research Council and the Australia and
Pacific Science Foundation; and to Tambussi from the
Fondo Nacional para la Investigación Científica y
Tecnológica (Argentina).

Biomechanical analysis of the
skull of the terror bird
Andalgalornis,
based on the engineering method of finite
element modeling. Simulation of stresses in
the skull that occur when holding onto prey
with the beak tip and pulling straight back
with the neck. Lower (safer) stress are
shown in bluer colors whereas higher (more
dangerous) stresses are shown with colors
toward the red end of the spectrum. Courtesy
of the Museo de La Plata and the University
of New South Wales.
QuickTime movie version.

Biomechanical analysis of the
skull of the terror bird
Andalgalornis,
based on the engineering method of finite
element modeling. Simulation of stresses in
the skull that occur when biting straight
down into prey with the beak like a hatchet.
This killing bite is more stressful for the
skull, but still is within safe limits.
Lower (safer) stress are shown in bluer
colors whereas higher (more dangerous)
stresses are shown with colors toward the
red end of the spectrum. Courtesy of the
Museo de La Plata and the University of New
South Wales.
QuickTime movie version.

Biomechanical analysis of the
skull of the terror bird
Andalgalornis,
based on the engineering method of finite
element modeling. Simulation of stresses in
the skull when shaking the beak from side to
side, as when thrashing small prey or when
grappling with large prey. This is a very
unlikely behavior because the stresses are
so high that fracture of the skull would
have been likely. Lower (safer) stress are
shown in bluer colors whereas higher (more
dangerous) stresses are shown with colors
toward the red end of the spectrum. Courtesy
of the Museo de La Plata and the University
of New South Wales.
QuickTime movie version.